Electrolytic production of iron



Patented Jan. 27, 1953 ELECTROLYTIC PRODUCTION OF IRON Clarence W. Balke, Highland Park, Ill., assignor to Fansteel Metallurgical Corporation, North Chicago, Ill., a corporation of New York No Drawing. Original application November 17,

1944, Serial No. 563,999. Divided and this application December 30, 1949, Serial No. 136,151

4 Claims.

This invention relates to the electrolytic preparation of iron from aqueous solutions and is particularly directed to the electrodeposition of the iron onto cathodes formed of specific metals.

In common electrodeposition processes such as copper plating, nickel plating and chromium plating methods, it is desired that the deposited metal adhere firmly to the cathode and ordinarily possess such characteristics that the deposited metal forms a protective or more or less permanent ornamental coating over the cathode. In theelectrolytic deposition of metals such as iron in which the deposited metal is to be treated to adapt it for powder metallurgy purposes, it is desired to deposit the metal upon a cathode from which it may be readily stripped or removed. The deposited metal should be of such character that it may be subsequently reduced to a powder of the required grain size. In the electrolytic refining of iron it is also desired that the deposited metal shall be adapted to be readily stripped or removed from the cathode.

Iron may be refined or prepared for powder metallurgy purposes by the employment of various types of electrolytes. The composition of the electrolyte depends upon the purpose of the electrolysis and the purpose for which the deposited metal-is to be used. In these methods one of the principal problems has been the stripping or removing of the deposited iron from the cathode. The cathode must be substantially inert in the electrolyte or of such composition so that under the conditions existing during electrolysis the deposited metal will not become contaminated with the material of the cathode. Various metals, such as copper, brass, low carbon steel, stainless steel and the like, have been employed in forming cathodes.

The degree of adherence of a deposited metal to the cathode will, of course, vary with cathodes formed of diiferent metals. For example, electrodeposition of iron and manganese on stainless steel results in the formation of a deposited layer with a fairly low degree of adherence providing the cathode surface is highly polished. In such instances the deposited metal may be removed by scraping, or bending or flexing the cathode plate. However, in order to obtain a deposit of low adherence it is essential that the cathode be given a high polish and in many instances be treated to more or less passivate'the metal surface before using the cathode. This necessitates considerable labor after stripping the deposited metal from the cathode and before the cathode is reimmersed in the electrolyte. In many instances small particles of the deposited metal adhere quite firmly to the cathode, probably due to slight pitting of the cathode. This deposited metal must be re moved by chemical or electrochemical means which further add to the labor and cost of maintaining the cathode surface in satisfactory condition. Although caution may be exercised in carefully and thoroughly cleaning and polishing the stainless steel cathode, some rather minute particles of deposited metal may remain on the oathode surfaces. These minute particles form nuclei upon which the metal deposits and forms a rather adherent deposit.

The principal object of this invention is to provide a method which employs a cathode material from which the deposited iron may be removed with a minimum effort.

A further object of this invention is to provide a method for the electrolytic preparation of iron powder suited for iron powder metallurgy.

Other objects and advantages of this invention will become apparent from the following description and claims.

The present invention contemplates the electrolysis of iron containing electrolytes with cathodes formed of certain of the refractory metals of the Fifth and Sixth Groups of the Periodic Table, namely, columbium, tantalum, molybdenum and tungsten. These metals remain substantially unaffected by the usual electrolytes and under the operating conditions, do not contaminate the electrolyte or the deposited metal and are relatively hard. They are, therefore, superior to known metals and common alloys in these respects.

The specific metal selected for the production of the cathode will be governed by economic fac tors as Well as by the characteristics desired in the deposited iron. The replacement cost of the cathodes may be disregarded because it appears that cathodes formed from these metals may be used indefinitely. The sole costs are, therefore, the initial cathode expense and the expense of stripping or removing the deposited metal from the cathode.

Cathodes formed of tantalum and columbium, for example, render stripping of the deposited iron from the cathode unnecessary in most instances. In the use of such cathodes the iron crystals, as they are deposited and grow, drop from the cathode and may be collected in a fabric bag or in a hopper positioned under the cathode.

For some purposes the iron powder so produced is of the proper degree of fineness, whereas for other purposes the iron so produced is too fine. At the present prevailing price of tantalum and columbium, the cost of large cathodes necessary for the commercial production of iron owder is prohibitive as compared to the savings in stripping and pulverizing costs.

Tungsten and molybdenum cathodes appear to be the most economical for commercial operations. Of these metals molybdenum is the most feasible. Molybdenum, like the other metals of this class, remains substantially unaffected by the electrolyte under the conditions of electrolysis. Molybdenum has a decided economical advantage over the other metals such as columbium, tantalum and tungsten because it may be produced in sheet form at a relatively low cost.

My invention may be illustrated by a description of the process as applied to the electrolysis of iron containing baths with a molybdenum cathode. It is to be understood that the specific illustration is not intended as a limitation of my invention.

Any desired bath may be employed in the deposition of the iron. For the preparation of iron for powder metallurgy purposes, the bath is preferably of such composition that relatively pure iron is deposited of such character that it is highly brittle and of a low ductility. The cathode current density may be of any desired value but is preferably sufiiciently high so as to aid in the deposition of brittle, relatively nonductile iron. Soluble or insoluble anodes may be employed but I prefer to employ soluble anodes consisting of substantially pure iron to avoid contamination of the bath. Scrap iron and steel is satisfactory, however, when steel is employed, the bath should be filtered occasionally to remove the carbon.

An electrolytic iron bath which has been found to be particularly well suited for the production of iron for powder metallurgy purposes comprises a solute of ferrous sulphate and ammonium sulphate in which the molecular quantity of ammonium sulphate substantially exceeds the molecular quantity of the ferrous sulphate, as described and claimed in my copending application, Serial No. 446,629, filed June 11, 1942, now abandoned.

It has been known to employ an aqueous solution of Mohrs salt as an electrolyte for the deposition of iron. Mohrs salt has a formula which corresponds to a ratio of 47.5 parts by weight of ammonium sulphate to 100 parts of hydrated ferrous sulphate, FESO4 7H2O, or 235.7 parts of ammonium sulphate to 100 parts of bivalent ferrous ion, Fe++. Substantial economies of power consumption can be attained by increasing the relative proportions of ammonium sulphate to about '70 parts to 100 parts by weight of ammonium sulphate to 100 parts of hydrated ferrous sulphate, or about 347.5 parts to 495.4 parts of ammonium sulphate to 100 parts of bivalent ferrous ion. In the preparation of iron powder in accordance with my invention, I prefer to employ baths in the upper portion of this range. The iron concentration may be between 25 grams and 40 grams of iron per liter of solution and I prefer to employ concentrated solutions, the only requirement being that the solution should not be too concentrated so as to cause crystallization of the salts.

For example, a bath was prepared using Mohrs salt dissolved in water to give an iron concentration of about 29 grams per liter. Electrolysis of this bath at a cathode current density of 125 amperes per square foot resulted in a power consumption of 0.87 kilowatt hour per pound of iron deposited. Increasing the ammonium sulphate from 47.5 parts to 71 parts per 100 parts of ferrous sulphate decreased the power consumption to 0.74 kilowatt hour per pound of iron deposited. A further increase of the amount of ammonium sulphate to parts per 100 parts of ferrous sulphate resulted in a further decrease in the power consumption to 0.72 kilowatt hour per pound of iron. Using Mohrs salt in an amount sufficient to give an iron concentration of about 35 grams per liter of solution and using a cathode current density of about 71 amperes per square foot required 0.69 kilowatt hour per pound of iron deposited. Increasing the relative amount of ammonium sulphate to 71 parts per 100 parts of ferrous sulphate decreased the power consumption to 0.63 kilowatt hour per pound of iron. When the ammonium sulphate was increased to 100 parts per 100 parts of ferrous sulphate, the power consumption decreased to 0.47 kilowatt hour per pound of iron deposited.

The bath preferably should be maintained slightly acid so as to result in some gassing at the cathode. It is believed that the metal deposited under such conditions is impregnated with small amounts of hydrogen which further tends to embrittle the deposited metal. Satisfactory results may be obtained by maintaining free sulphuric acid in the bath to the extent of a molarity of 0.002 to 0.004, or to maintain the bath at a pH value of about 3.0. Since the acid becomes depleted during electrolysis it is neces sary to replenish the acid periodically.

Depositions of iron of the desired characteristics may be obtained by employing current densities of from about 20 amperes to about 500 amperes per square foot of cathode surface. I prefer to operate at cathode current densities between about 30 amperes and 70 amperes persquare foot. The higher current densities require a greater electrical energy consumption per unit weight of iron deposited and will result in a greater heating of the electrolyte. When using the high current densities it may be necessary to cool the electrolyte since I have found that the iron deposited from electrolytes maintained at temperatures below 50 C. is more desirable.

Electrolytic iron baths in accordance with the invention claimed herein which I have found to be highly satisfactory for the production of electrolytic iron for powder metallurgy purposes are described and claimed in my copending application Serial No. 564,000, filed November 17, 1944, now Patent No. 2,464,168, granted March 8, 1949. The electrolyte in these baths comprises a solute of ferrous chloride, FeC1z.4H-2O, ammonium sulphate, (NH4)2SO4, and ferrous sulphate FeSO4.7H2O

in which these salts are in the ratio by weight of from 1:7:9 to 1:23:32. The total iron content of the electrolyte may vary between about 25 grams to about 40 grams per liter of electrolyte. The pH of the electrolyte is preferably maintained at about 3.0 or slightly below the hydrogen ion concentration at which the ferric hydroxide would be precipitated.

Erample 1 A bath was prepared having the following composition per liter of solution:

Grams Ferrous chloride (FeClzAI-IzO) 18 Ammonium sulphate Ferrous sulphate (FeSO4.7HzO) Mineral acids such as hydrochloric acid or sulphuric acid may be added to lower the pH of the solution to about 3.0 and, asthebath is operated, additional quantities may be added from time to time to maintain the electrolyte at such value.

Example 2 An electrolyte prepared having the following composition per liter of solution:

Grams Ferrous chloride (FEC1L4H20) 4.5 Ammonium sulphate 101 Ferrous sulphate (FeSO-a'YI-IzO) 142 This bath was brought to a pH of about 3.0 by the addition of sulphuric-acid and was maintained at such value by periodic additions of acid.

The cathode current density may be varied over a wide range and I have used current densities up to about 375 amperes per square foot; In the operation of this electrolyte I prefer to employ cathode current densities between about 30 amperes and '70 amperes per square foot. I prefer to maintain the temperature of the electrolyte below about 50 C., which further aids in the production of a brittle deposit or iron. At these preferred current densities it is usually not necessary to cool the electrolyte. Electrolysis of electrolytes having compositions and concentrations within the ranges set forth above at cathode current 'metals. appear t be substantially unaffected by the deposition of the iron and immediately after stripping the deposited iron from the cathode the densities of from 70 amperes to about 85 amperes per square foot have resulted in power consumptions ranging from about 0.48 to about 0.8 kilowatt hour per pound of iron deposited.

Ferrous chloride baths may also be employed for the deposition of iron for iron powder metallurgy purposes. I have produced satisfactory deposits of iron for such purposes from baths containing from 260 grams to 450 grams ferrous chloride FBC12.4H2O, per liter of solution operated at current densities as high as 500 amperes per square foot. Baths within the lower portion of this range and operated at cathode current densities of about '70 amperes per square foot result in a power consumption of about 0.5 kilowatt hour per pound of iron produced. Other chloride baths, such as baths containing sufficient amounts of ferrous chloride and ammonium chloride equivalent to the double salt 2NH4Cl.FeC1z.4I-I2O have produced similar results.

After the desired amount of iron has been deposited upon the cathode, the cathode is removed from the bath, washed free of electrolyte and dried. The iron deposit on the molybdenum cathode may be easily removed by merely scraping or flexing the cathode. In most instances the deposited iron may be removed from the cathode by merely inserting a thin blade between the deposit and the cathode surface. The deposited iron has a very low degree of adherence to the cathode and is held on the cathode principally by the portion of the deposited material which looks around the edges of the cathode. A slight blow with a hammer is sufficient to fracture the deposited iron on the surface of the cathode. Iron which clings around the edges of the cathode may be removed by a slight blow or in many cases be stripped from the cathode by manually pulling the deposited metal from the edges of the cathode.

The iron may then be reduced to the desired powder form or particle size in any desired manner. For example, the broken deposit may be crushed to disintegrate the pieces into a granular or'pellet form. The granular material is then cathode may be again immersed in the bath without a cleansing or polishing operation. The condition of the surface does not appear to affect the ease with which the deposited iron may be removed. In the practice of this invention, molybdenum sheets, for example, formed by rolling without further polishing of the surfaces have been highly satisfactory. Such plates after several months use appear to remain unaffected and about the only noticeable change in the surface condition is the surface scratches resulting from the use of a scraping blade inserted between the cathode surfaceanrl the deposited iron used to fracture the deposited iron plate.

Where the deposited iron is intended for powder metallurgy purposes, it is desirable to obtain as brittle a deposit as possible. It is not necessary to maintain conditions whereby extremely brittle and friable deposits are produced if the electrolysis is employed for refining iron. In such instances, slight brittleness is desired to aid in fracturing the deposit in removing the iron from the cathode and the most economical refining of the metal is of primary importance with no particular regard to the ductility of the deposit.

The electrolysis of iron containing solutions with cathodes formed of the specific metals set forth herein entirely eliminates the necessity of surface polishing of the cathode surface before depositing iron on the cathode. These cathodes permit the stripping or removal of the deposited metal with a minimum effort.

This application is a division of my copending application Serial No. 563,999, filed November 1'7, 1944, now abandoned.

I claim:

1. The method of producing electrolytic iron for powder metallurgy purposes which comprises forming an aqueous bath containing ferrous chloride, ammonium sulphate and ferrous sulphate in the ratio by weight of from about 127:9 to about 1:23:32, electrodepositing the iron onto a cathode formed of a metal selected from the group consisting of columbium, tantalum, molybdenum and tungsten, removing the deposited iron from the cathode and reducing the iron to powder form. the aqueous bath containing, in solution, from about 25 grams to about 40 grams of iron per liter and a suffi-cient amount of acid to maintain the pH slightly below the hydrogen ion concentration at which ferric hydroxide is precipitated.

2. The method of producing electrolytic iron for powder metallurgy purposes which comprises forming an aqueous bath containing ferrous chloride, ammonium sulphate and ferrous sulphate in the ratio by Weight of from about 1:7 :9 to about 1:23:32, electrodepositing the iron onto a molybdenum cathode, removing the deposited iron from the cathode and reducing theiron to powder form, the aqueous bath containing, in solution, from about 25 grams to about 40 grams of iron per liter and a sufllcient amount of acid to maintain the bath at a pH of about 3.0.

3. The method of producing eelctrolytic iron for powder metallurgy purposes which comprises providing an aqueous bath containing ferrous chloride, ammonium sulphate and ferrous sulphate in the ratio by weight of from about 1:7:9

to about 1:23:32, immersing a cathode formed of a metal selected from the group consisting of columbium', tantalum, molybdenum and tungsten in the bath, subjecting the bath to electrolysis at a cathode current density of at least 20 amperes per square foot while maintaining the temperature of the bath below 50 C.,'removing the deposited iron from the cathode and reducing the iron to powder form, the aqueous .bath containing, in solution, from about 25 grams to about 40 grams of iron per liter and a suflicient amount of acid to maintain the bath at a pH of about 3.0.

4. The method of producing electrolytic iron for powder metallur y Purposes which comprises forming an aqueous bath containing ferrous chloride, ammonium sulphate and ferrous sulphate in the ratio by weight of from about 1:719 to about 1:23: 32, immersing a molybdenum cathode in the bath, subjecting the bath to electrolysis at a cathode current density of at least 20 amperes per square foot while maintaining the temperature of the bath below 50 C., removing the deposited iron from the cathode and reducing the iron to powder form, the aqueous bath containing, in solution, from about 25 grams to about 40 grams of iron per liter and a sufficient amount of acid to maintain the bath at a pH of about 3.0.

CLARENCE W. BALKE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Transactions of the Electrochemical Society, vol. 25, 1914, pages 529-536. 

1. THE METHOD OF PRODUCING ELECTROLYTIC IRON FOR POWDER METALLURGY PURPOSES WHICH COMPRISES FORMING AN AQUEOUS BATH CONTAINING FERROUS CHLORIDE, AMMONIUM SULPHATE AND FERROUS SULPHATE IN THE RATIO BY WEIGHT OF FROM ABOUT 1:7:9 TO ABOUT 1:23:32, ELECTRODEPOSITING THR IRON ONTO A CATHODE FORMED OF A METAL SELECTED FROM THE GROUP CONSISTING OF COLUMBIUM, TANTALUM, MOLYBDENUM AND TUNGSTEN, REMOVING THE DEPOSITED IRON FROM THE CATHODE AND REDUCING THE IRON TO POWDER FORM, THE AQUEOUS BATH CONTAINING, IN SOLUTION, FROM ABOUT 25 GRAMS TO ABOUT 40 GRAMS OF IRON PER LITER AND A SUFFICIENT AMOUNT OF ACID TO MAINTAIN THE PH SLIGHTLY BELOW THE HYDROGEN ION CONCENTRATION AT WHICH FERRIC HYDROXIDE IS PRECIPITATED. 